Observations of cross‐channel structure of flow in an energetic tidal channel

Measurements of velocity and density profiles were made to describe the transverse structure of flow in Chacao Channel, Southern Chile (41.75°S), where typical tidal velocities are ∼4 m/s. Current profiles were obtained with a 307.2 kHz Acoustic Doppler Current Profiler (ADCP) over 25 repetitions of a cross‐channel transect during one semidiurnal tidal cycle. The 2.2 km long transect ran northeast/southwest across the channel. A northern channel (120 m deep) and a southern channel (85 m deep) were separated by Remolinos Rock, a pinnacle that rises to 20 m depth at ∼0.7 km from the southern side. Density measurements to depths of ∼50 m were obtained with a Conductivity, Temperature, and Depth (CTD) recorder at the north and south ends of each transect repetition. One CTD profile was also taken in the middle of the northern channel. The mean flow exhibited weak vertical structure because of strong vertical mixing. The predominant lateral structure consisted of mean outflow (toward the ocean) in the channels and mean inflow (toward Gulf of Ancud) over the pinnacle and the sides of the channel. This lateral structure pattern was consistent with the mean flow pattern expected from tidal rectification, as robust overtides were generated throughout the transect. The contributions to flow divergence and vorticity by the lateral variations of the lateral flow (∂ v /∂ y ) and by the lateral shears of the along‐channel flow (∂ u /∂ y ) , respectively, were both of the order of 10 −3 s −1 . This caused advective and frictional forces (both horizontal and vertical) to be dominant in the across‐channel momentum balance, as they were more than twenty times the Coriolis acceleration. The present work then represents one of the few examples reported where lateral friction (proportional to ∂ 2 v /∂ y 2 ) appears relevant to the transverse momentum balance.